Securities (Details 2)	3 Months Ended
	Mar. 31, 2017 USD ($) security	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Fair Value
Less Than 12 Months	$ 2,114,479,000		$ 2,162,896,000
12 Months or More	195,679,000		253,038,000
Total	2,310,158,000		2,415,934,000
Gross Unrealized Losses
Less Than 12 Months	(46,427,000)		(51,760,000)
12 Months or More	(4,119,000)		(4,581,000)
Total	(50,546,000)		$ (56,341,000)
Impairment loss	$ 0	$ 0
Number of securities in an unrealized loss position | security	163		170
Fair Value
Less Than 12 Months	$ 2,114,479,000		$ 2,162,896,000
12 Months or More	195,679,000		253,038,000
Total	2,310,158,000		2,415,934,000
Gross Unrealized Losses
Less Than 12 Months	(46,427,000)		(51,760,000)
12 Months or More	(4,119,000)		(4,581,000)
Total	(50,546,000)		(56,341,000)
U.S. Treasury securities
Fair Value
Less Than 12 Months	680,801,000		670,268,000
12 Months or More	0		0
Total	680,801,000		670,268,000
Gross Unrealized Losses
Less Than 12 Months	(8,482,000)		(9,829,000)
12 Months or More	0		0
Total	$ (8,482,000)		$ (9,829,000)
Number of securities in an unrealized loss position | security	27		26
U.S. government agency and U.S. government sponsored enterprise debt securities
Fair Value
Less Than 12 Months	$ 154,847,000		$ 203,901,000
12 Months or More	0		0
Total	154,847,000		203,901,000
Gross Unrealized Losses
Less Than 12 Months	(2,876,000)		(3,249,000)
12 Months or More	0		0
Total	$ (2,876,000)		$ (3,249,000)
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities
Gross Unrealized Losses
Number of securities in an unrealized loss position | security	83		82
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	$ 212,706,000		$ 202,106,000
12 Months or More	35,193,000		29,201,000
Total	247,899,000		231,307,000
Gross Unrealized Losses
Less Than 12 Months	(5,879,000)		(5,452,000)
12 Months or More	(961,000)		(766,000)
Total	(6,840,000)		(6,218,000)
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Residential mortgage-backed securities
Fair Value
Less Than 12 Months	598,526,000		629,324,000
12 Months or More	132,328,000		119,603,000
Total	730,854,000		748,927,000
Gross Unrealized Losses
Less Than 12 Months	(8,151,000)		(9,594,000)
12 Months or More	(1,331,000)		(1,955,000)
Total	(9,482,000)		(11,549,000)
Municipal securities
Fair Value
Less Than 12 Months	45,327,000		57,655,000
12 Months or More	6,925,000		2,692,000
Total	52,252,000		60,347,000
Gross Unrealized Losses
Less Than 12 Months	(1,006,000)		(1,699,000)
12 Months or More	(393,000)		(201,000)
Total	(1,399,000)		(1,900,000)
Non-agency residential mortgage-backed securities | Investment grade
Fair Value
Less Than 12 Months	10,729,000		5,033,000
12 Months or More	0		6,444,000
Total	10,729,000		11,477,000
Gross Unrealized Losses
Less Than 12 Months	(107,000)		(101,000)
12 Months or More	0		(14,000)
Total	(107,000)		(115,000)
Corporate debt securities | Investment grade
Fair Value
Less Than 12 Months	0		0
12 Months or More	2,254,000		71,667,000
Total	2,254,000		71,667,000
Gross Unrealized Losses
Less Than 12 Months	0		0
12 Months or More	(222,000)		(375,000)
Total	(222,000)		(375,000)
Corporate debt securities | Non-investment grade
Fair Value
Less Than 12 Months	0		0
12 Months or More	9,184,000		9,173,000
Total	9,184,000		9,173,000
Gross Unrealized Losses
Less Than 12 Months	0		0
12 Months or More	(1,007,000)		(1,018,000)
Total	(1,007,000)		(1,018,000)
Foreign bonds | Investment grade
Fair Value
Less Than 12 Months	380,530,000		363,618,000
12 Months or More	9,795,000		14,258,000
Total	390,325,000		377,876,000
Gross Unrealized Losses
Less Than 12 Months	(19,409,000)		(21,327,000)
12 Months or More	(205,000)		(252,000)
Total	$ (19,614,000)		$ (21,579,000)
Number of securities in an unrealized loss position | security	13		13
Other securities
Fair Value
Less Than 12 Months	$ 31,013,000		$ 30,991,000
12 Months or More	0		0
Total	31,013,000		30,991,000
Gross Unrealized Losses
Less Than 12 Months	(517,000)		(509,000)
12 Months or More	0		0
Total	(517,000)		(509,000)
Non-agency commercial mortgage-backed security
Fair Value
Less Than 12 Months	0		0
12 Months or More	0		0
Total	0		0
Gross Unrealized Losses
Less Than 12 Months	0		0
12 Months or More	0		0
Total	$ 0		$ 0